It has long been an object of silver halide-based color photographic materials to reproduce colors in a desired manner in terms of hue saturation. In practice, the reproduction of color by such materials is limited in several ways. First, the sensitivity of the silver halide emulsions to a desired single light color is not perfect and they will therefore absorb some amount of light of undesired color. This leads to formation of dye in the wrong color record resulting in less pure hues. For example, the red sensitivity of the emulsions generally occurs at longer wavelengths than the human eye. If the red sensitivity of the film is moved closer to the eye maximum sensitivity, its sensitivity to green light also increases. Thus in such situations, the red sensitive layer is partially exposed during green light exposures leading to the formation of some cyan dye along with magenta dye. This alters the hue of the image and decreases its saturation. Second, the image dyes formed are not perfect in hue and have unwanted side absorbances. Thus, some density in the unwanted color regions it formed in addition to the desired density, again degrading color saturation. Finally in some circumstances, it is desirable to increase color saturation to a greater degree than the actual image in order to make the image visually more pleasing.
It is well known too that color reproduction of such materials can be partially controlled by the use of imagewise development inhibitor releasing (DIR) couplers. During development, DIR couplers react with oxidized developer to release an inhibitor fragment or a precursor of an inhibitor fragment which can diffuse out of that layer and into a different color record where inhibition occurs. This has the overall effect of reducing the amount of dye formed in one color record as a function of exposure of another and can effectively be used to manipulate hue and increase color saturation. This process is called interimage. For example, a film with a DIR coupler in the green layer and given a mostly green exposure will cause a decrease in development in the red record due to the action of the inhibitor released in the green. This causes less cyan dye to be formed than when the inhibitor was not present. The final green image will have less red density and its overall saturation will be increased. It should be noted that all possible colors are not weighted equally in tennis of creating a pleasing overall image and that the reproduction of some key colors (for example, flesh tones, green grass, blue sky, etc.) is more important than others.
The creation of interimage effects with DIR couplers as currently practiced is deficient in a number of ways. First, the inhibitor fragment (or precursor) released from the DIR coupler is fire to diffuse in all directions. Thus, the inhibitor can affect both of the other color records, even if it was desired to affect only one. For example, putting the DIR coupler in the green record will decrease the amount of blue development as well as the red. The amount of interimage effects on the blue and red records from the green are linked and cannot be manipulated separately. This non-specificity of interimage effects limits the ability to control and manipulate color reproduction of the key colors. Second, the fragment released from the DIR will also cause inhibition in the layer in which it is released. This can lead to over-inhibition of the layer in which the DIR coupler is located resulting in low contrast and a loss of sensitivity to light, particularly with strong inhibitor fragments. It is possible to avoid this in part by using milder inhibitors or by using timing groups to delay the introduction of the free inhibitor fragment. In such situations, the diffusion pathlength of the inhibitor fragment is increased and seasoning of the fragments into the developer becomes a problem. In order to avoid these seasoning effects, mild inhibitor fragments often have a hydrolyzable substituent which, upon hydrolysis in the developer solution, renders them inactive after a period of time. Examples are shown in U.S. Pat. No. 4,782,012, U.S. Pat. No. 4,477,563, U.S. Pat. No. 4,937,179, U.S. Pat. No. 5,004,677, DE-A 3909486, DEA-3209486, EP-A-167,168, EP-A-488,310, EP-A-440,466 and EP-A-219,173.
Couplers with potential silver absorbing or complexing groups are known. U.S. Pat. No. 2,353,754, U.S. Pat. No. 2,756,142, U.S. Pat. No. 2,308,023, U.S. Pat. No. 2,296,306, U.S. Pat No. 2,289,803, U.S. Pat. No. 2,412,700 and FR 1459811 all describe the use of color couplers that form insoluble silver salts to immobilize them in photographic systems. These couplers rely solely on the salt formation to prevent diffusion and are not additionally ballasted or substituted with additional anti-diffusion groups. All of the examples contained in these patents have ClogP (defined hereinafter) no higher than 4.25 indicating low hydrophobicity. Such materials tend to desensitize the silver emulsions to light and are inhibitors of silver development themselves.
DE 1 95 31569 A1 describes couplers that improve granularity that bear acidic residues that impart diffusively in an alkane medium as well as couplers that bear a precursor to a thiol group. Of the examples shown, only GB-3 (ClogP=10.46), PP-1 (ClogP=6.59) and PP-4 (ClogP=17.59) have groups with an --N--H or --S--H group and a ClogP greater than 6.25.
U.S. Pat. No. 5,158,864 describes couplers with silver absorbing or complexing groups to reduce the sensitivity/granularity ratio. These couplers must have a certain reactivity, have a preferred upper concentration limit of 1 mmol per mole of silver with an absolute upper limit of 10 mmol per mole silver and must not contain a diffusion inhibiting ballast residue. They are described as being soluble to some extent in aqueous media. Of 48 examples, the average ClogP is 3.09 with only HK11 (ClogP=10.87), HK16 (ClogP=6.72), HK28 (ClogP=6.28) and HK31 (ClogP=8.53)having a ClogP greater than 6.25.
U.S. Pat. Nos. 5,441,857 and 5,622,817 (describe ACR couplers, which are couplers that upon reaction with oxidized developer release a second coupler bearing a silver absorbing or complexing group to increase sensitivity. In these materials, the second coupler does not contain a ballast or anti-diffusion group nor is the silver absorbing or complexing group free to interact with the silver until after development and coupling of the first coupler. However, in the '857 reference, examples I-5 (ClogP=14.42), I-7 (ClogP=13.46) and I-8 (ClogP=8.60) contain groups with N--H or S--H bonds that could interact with silver.
A problem to be solved is to provide a color photographic element that will provide improved color reproduction.